Rising star Marco Rubio faced a barrage of attacks in the latest Republican debate, with rivals challenging his readiness to be president and the depth of his expertise as they sought to salvage their own White House hopes.

Rising star Marco Rubio faced a barrage of attacks in the latest Republican debate, with rivals challenging his readiness to be president and the depth of his expertise as they sought to salvage their own White House hopes.

Senator Ted Cruz, fresh from his victory in Iowa, also came under heavy criticism for controversial political tactics, with one candidate disparaging him for having "Washington ethics" and being willing to test the campaign's legal limits.

The focus on the two senators allowed Republican frontrunner Donald Trump to go largely untouched in his return to the debate stage in Manchester, New Hampshire.

His grip on the Republican lead has been shaken by his second-place finish in Iowa, though the next contest on Tuesday in New Hampshire is still his to lose.

In the race for the Democratic presidential nomination, former secretary of state Hillary Clinton is facing a strong challenge from liberal senator Bernie Sanders.

New Hampshire's primary could further narrow an already shrinking Republican field.

Hard-fought, expensive and far-ranging, the campaign has become a fight for the future of the party, though the direction it will ultimately take remains deeply uncertain.

Florida's Senator Rubio has sought to appeal both to mainstream Republicans and those eager to change the status quo.

But his rivals, particularly New Jersey governor Chris Christie, have been blistering in their criticism of what they see as his slim qualifications to serve as commander in chief.

"You have not been involved in a consequential decision where you had to be held accountable," Mr Christie said. "You just simply haven't."

Mr Christie, as well as former Florida governor Jeb Bush and Ohio governor John Kasich, has staked his campaign on New Hampshire, pouring most of his resources into the state in recent weeks.

All three played a more substantial role in this debate than in earlier contests, though each is still likely to face intense pressure to end his campaign if he is unable to pull off a strong finish in New Hampshire.

Mr Christie has built his closing argument around his criticism of Mr Rubio, and he kept up that approach on the debate stage.

He accused the senator of being a candidate governed by talking points - then pounced when Mr Rubio played into his hands by repeating multiple times what appeared to be a planned response to criticisms about his qualifications.

"That's what Washington, DC, does," Mr Christie said. "The drive-by shot at the beginning with incorrect and incomplete information and then the memorised 25-second speech that is exactly what his advisers gave him."

Mr Rubio has sought to deflect criticism of his relative inexperience and the comparisons it draws to President Barack Obama.

He argues the problem with the president is not that he is naive, but that he is pushing an ideology that hurts the country. He made that point repeatedly throughout the debate.

Mr Rubio wavered in defending his decision to walk away from the sweeping immigration bill he originally backed in the Senate - perhaps the legislation he is most closely associated with - and said he would not pursue similar measures as president.

"We can't get that legislation passed," Mr Rubio said of the bill that would have provided a pathway to citizenship for millions of people in the US illegally.

The senator found his footing later in the debate when outlining his call for more aggressive action to fight the Islamic State and emphasising his anti-abortion stance.

Mr Cruz was the victor in Iowa, triumphing over billionaire Mr Trump by drawing heavily on the support of evangelical voters.

But he has faced criticism for messages his campaign sent to voters ahead of the caucuses saying rival Ben Carson - another favourite of religious conservatives - was dropping out and urging the retired neurosurgeon's supporters to back him instead.

Mr Cruz apologised for his campaign's actions, but not before Mr Carson attacked him for having "Washington ethics".

Such ethics " say if it's legal, you do what you do to win", Mr Carson claimed.

Mr Trump was back on the debate stage after skipping the last contest before the Iowa caucuses.

After spending the past few days disputing his second-place finish in Iowa, he sought to refocus on the core messages of his campaign.

They include blocking Muslims from coming to the US and deporting all people in the country illegally, while maintaining he has the temperament to serve as president.